In Internet Protocol Multimedia Subsystem (IMS) networks, a terminal is required to register IMS public user identifiers to the IMS network in order to receive terminating calls for the IMS public user identifiers. The IMS also uses application servers (AS) to assist in providing services to IMS users. As such, the registration status of an IMS public user identifier is an indication of whether the network views that the user is reachable or not. As such, the Registration status impacts the application logic of application servers, and also which application servers can be invoked.
In addition to performing the initial Registration to the IMS network, a termination is required to periodically refresh the registration state in the network via performing the registration again. The terminal may also cancel the registration (e.g. before powering down). It is possible to administratively de-register a user from the network.
As such, a home subscriber server (HSS) recognizes that a user may be:                Registered: A Public User Identity is Registered at the request of the user and has an S-CSCF assigned.        Un-Registered: A public User Identity is not registered but has a S-CSCF assigned to execute Unregistered state services as a consequence of a terminating call or there is a S-CSCF keep the profile stored        Not-Registered: Public Identity is not Registered and has no S-CSCF assigned.        
System and service aspects of IMS (among other registration and de-registration procedures) are described in Third Generation Partnership Project (3GPP) Technical Specification (TS) 23.228 V7.5.0 (September 2006). A more detailed description of the service logic for some of these service aspects and procedures is given in 3GPP TS 29.228 V7.3.0 (September 2006).
Whether the Public user identity is Registered or Un-Registered/Not Registered may impact which application servers are invoked as different Filter Criteria is applied, and/or the behavior of the application logic within the application servers. This can imply that the expected behavior of an application server is different when the call originates from the circuit switch (CS) domain or the IMS domain, in that when originating/terminating a call to/from a packet switch (PS) access over IMS (normal access to IMS), the application servers will see that the user is Registered and apply registered services, however when originating/terminating a call via a CS access, the user may be unregistered.
One example is in the case of a terminating call for a voice call continuity (VCC) user. If the user is registered, the application logic is likely to try to deliver the call to the user, however if the user is not registered, the application logic is likely to forward the call to, e.g., a mail box. However, applications are appearing where a call may be made to or from a user, where the terminal for the user has not performed a registration. One example of this is VCC which is being progressed within 3GPP. TS 23.206 describes a VCC user entity (UE) that can originate and terminate a call without being registered to the IMS.